1. Field of the Invention
The present invention relates to a mobile communication technology, and more particularly, to a method of efficiently transmitting and receiving downlink control information (DCI).
2. Discussion of the Related Art
In a next-generation mobile communication system, a high data transmission rate is required. Thus, various researches into a multi-input multi-output (MIMO) antenna technology are ongoing. First, a general MIMO technology will be briefly described.
The MIMO is abbreviated from the term “multi-input multi-output” and indicates a method of employing multiple transmission antennas and multiple reception antennas so as to improve transmission/reception data efficiency, instead of a conventional method using one transmission antenna and one reception antenna. That is, a transmitter or a receiver of a radio communication system uses multiple antennas so as to increase communication capacity or improve transmission/reception performance. Hereinafter, the “MIMO” is also referred to as “multi-input multi-output antenna”.
The multi-input multi-output antenna technology indicates a technology of collecting data pieces received via several antennas without depending on a single antenna path in order to receive one message. According to the MIMO technology, a data transmission rate is improved in a specific range or a system range can be increased with respect to a specific data transmission rate.
Since next-generation mobile communication requires a data transmission rate which is significantly higher than that of the existing mobile communication, an efficient multi-input multi-output antenna technology is expected to be necessarily required. Under these circumferences, the MIMO communication technology is the next-generation mobile communication technology which is widely applicable to mobile communication terminals and repeaters. The MIMO technology is attracting attention as the next-generation technology to overcome the restricted transmission amount of the mobile communication that has reached the limit due to the data communication extension.
Among various technologies of improving transmission efficiency which are currently being researched, the MIMO technology of using multiple antennas in both a transmitter and a receiver is attracting most attention as a method of remarkably improving communication capacity and transmission/reception performance with increasing additional frequency allocation or power consumption.
FIG. 1 is a view showing the configuration of a general MIMO antenna communication system.
As shown in FIG. 1, if the number of transmission antennas is increased to NT and, at the same time, the number of reception antennas is increased to NR, a theoretical channel transmission capacity is increased in proportion to the number of antenna ports, unlike the case where multiple antennas are used in only any one of the transmitter and the receiver. Thus, frequency efficiency can be remarkably improved. The transmission rate due to the increase in channel transmission capacity can be theoretically increased by a value obtained by multiplying a maximum transmission rate Ro at the time of using one antenna by a rate increasing ratio (Ri).Ri=min(NT,NR)  Equation 1
That is, for example, in an MIMO communication technology using four transmission antennas and four reception antennas, the transmission rate is theoretically four times that of a single-input single-output antenna system.
After the theoretical increase in the capacity of the MIMO antenna system was proved in the mid-1990s, various technologies of substantially improving a data transmission rate have been actively developed up to now. Among them, several technologies have been already applied to the various radio communication standards such as the third-generation mobile communication and the next-generation wireless local area network (LAN).
According to the trend of the research into the MIMO antenna up to now, various researches such as researches into information theory related to the computation of the communication capacity of a MIMO antenna in various channel environments and multiple access environments, researches into the model and the measurement of the radio channels of the MIMO system, and researches into space-time signal processing technologies of improving transmission reliability and transmission rate have been actively conducted.
The MIMO technology includes a spatial diversity method for increasing transmission reliability using symbols passing through various channel paths and a spatial multiplexing method for improving a transmission rate by simultaneously transmitting a plurality of data symbols using a plurality of transmission antennas. Recently, researches into a method of obtaining the respective advantages of the two methods by combining the two above-described methods are ongoing.
Hereinafter, the methods will be described in detail.
First, the spatial diversity method includes a space-time block coding method and a space-time trellis coding method using both a diversity gain and a coding gain. Generally, the trellis coding method is excellent in view of the improvement of a bit error rate and the degree of freedom for code generation, but the space-time block coding method is advantageous in that computation complexity is simple. A spatial diversity gain can be obtained from a product NT×NR of the number NT of transmission antennas and the number NR of reception antennas.
Second, the spatial multiplexing method indicates a method of transmitting different data streams via transmission antennas. At this time, in a receiver, mutual interference is generated between data which are transmitted from a transmitter. The receiver eliminates the interference using an adequate signal processing method and receives the data. The receiver for eliminating noise, which is used herein, includes a maximum likelihood receiver, a zero forcing (ZF) receiver, a minimum mean-squared errors (MMSE) receiver, a Diagonal Bell Laboratories Layered Space-Time (D-BLAST) receiver and a Vertical Bell Laboratory Layered Space-Time (V-BLAST) receiver. In particular, if the transmitter can know channel information, a singular value decomposition (SVD) method may be used.
Third, a combination of the spatial diversity method and the spatial multiplexing method may be used. If only the spatial diversity gain is obtained, a performance improvement gain according to the increase in diversity order is gradually saturated. If only the spatial multiplexing gain is obtained, the transmission reliability of the radio channel deteriorates. Accordingly, researches into the methods of obtaining both the two gains while solving the above-described problems have been conducted. Among them, a Double Space-Time Transmit Diversity (Double-STTD) or Space-Time Bit Interleaved Coded Modulation (STBICM) may be used.
In the MIMO antenna system, the transmitter performs precoding with respect to transmission data and transmits the pre-coded data, and the receiver receives the signal using a precoding vector or a precoding matrix used in the transmitter.
The precoding matrix for performing the precoding uses a specific precoding matrix among precoding matrixes which are predefined in the form of a codebook in both the transmitter and the receiver. That is, the receiver feeds back channel information according to the specific precoding matrix in the predefined codebook to the transmitter, and the transmitter transmits the signal using the feedback signal.
In downlink transmission, the receiver may be a user equipment (UE) or a terminal and the transmitter may be a base station, a node-B or an eNode-B (hereinafter collectively called as “base station”). For example, the UE may report a specific precoding matrix index (hereinafter, referred to as a “PMI”) in the predefined codebook via an uplink channel, and the base station may transmit a downlink signal using a precoding matrix corresponding to the reported PMI.
There may be a plurality of PMIs which are temporally reported by the UE. Accordingly, confusion about which of the PMIs is used by the base station may occur. In order to solve this problem, the base station preferably transmits control information indicating which of the PMIs reported by the UE is used. If the control information explicitly indicating which of the PMIs reported by the UE is used by the base station is transmitted via a downlink channel, the overhead of the downlink control information may be increased.
Accordingly, there is a need for a technology of efficiently preventing confusion about a PMI used between the base station and the UE from occurring using a small amount of control information.